题目

如图所示，小阳同学利用滑轮组提升重170N的物块，每个滑轮重为10N(不计轮与轴的摩及绳重)。小阳在匀速拉动绳子的过程中，6s内物块上升了3m，则该6s内（ ）A.小阳做的有用功为560J B.小阳对绳子的拉力为70NC.小阳做功的功率为90W D.该滑轮组的机械效率为84% 答案：【答案】C【解析】A．小阳做的有用功为故A不符合题意；B．由图可知，滑轮组中的n=3，小阳对绳子的拉力为故B不符合题意；C．小阳做功的功率为故C符合题意；D．小阳做功为该滑轮组的机械效率为故D不符合题意。故选C。B      An earthquake happened in Ya an,Sichuan,on April 20th,2013. Nearly 200 people died and at least ten thousand people were injured in the earthquake. Moreover,several towns were severely destroyed. The major roads to Ya an were cut off. The electricity and the telecommunication services were broken off in many parts of Ya’an.     But we can see examples of love everywhere. For example,an eight-year-old boy used his hands to dig out his younger sister under the rock and sand. People in Ya’an never gave up and kept on finding their relatives until the last minute.     Living in the safe Hong Kong,we haven’t had any disaster like the earthquake. We get love from our families and friends,so we are very lucky. So,we should cherish (珍爱) what we have now. Love everyone around you and give praise for their love. Also,we have learnt that time is limited,and the resolution is key in our lives. From now on,we should pay more attention to pursuing (追求) our dreams while we have the opportunity,or we might regret it. We never know when a disaster may take away our lives.() 1. How many people were injured in the earthquake?   A. At least 200.   B. Less than 200.   C. At least 10 ,000.   D. Less than 10 ,000.() 2. Where does the writer live?   A. In Sichuan. B. In Hong Kong. C. In Yaan. D. In Beijing.() 3. The earthquake led to the following except that .   A. the traffic to Ya an stopped   B. people couldn’t contact those outside   C. some people lost their homes   D. flood washed away many villages() 4. What does the underlined word “disaster” mean in Chinese?   A. 灾难   B. 负担   C. 紧急事件   D. 危机() 5. From the disaster in Ya’an,Sichuan,we have learnt .   A. we should make our own decisions on everything   B. we should enjoy life as much as we can   C. we know when a disaster will take away our lives   D. we should love everyone around us and give praise for their love